This invention relates to thermal insulation for the suspension system for a superconducting magnet assembly for a recondensing magnetic resonance imager (hereinafter called "MRI") and in particular to non-uniform multilayer insulation blankets for use in such magnet assemblies.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen cryocoolant means. The extreme cold ensures that the magnet coils are superconducting, such that the coils can be operated in the persistent mode, that is when a power source is initially connected to the coil for a relatively short period of time to introduce a current flow through the coils and a superconducting switch is then closed, the current will continue to flow, thereby maintaining the large coil current and a magnetic imaging field. Superconducting magnets find wide application in the field of MRI.
Considerable research and development efforts have been directed at reducing magnet heat load and eliminating the need to continuously replenish the boiling liquid helium. While the use of liquid helium to provide cryogenic temperatures has been widely practiced and is satisfactory for MRI operation, the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly. This has led to considerable effort being directed at superconducting materials and magnet structures in which the helium is recondensed and reused. This in turn leads to the need for good thermal insulation between the cryostat and the ambient temperature, a temperature difference in the order of over 400.degree. F.
Another problem encountered by most MRI equipments is that they utilize solenoidal magnets enclosed in cylindrical structures with a central axial bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm bore, which can induce claustrophobia in some patients. The desirability of an open architecture structure in which the patient is not essentially totally enclosed has long been recognized. Unfortunately, an open architecture structure poses a number of technical problems and challenges. One problem is to provide a suitable support structure with insulation which occupies much less space than conventional support structures and insulation, and yet which nevertheless can support the magnet assembly under the considerable electromagnetic forces and thermal forces encountered during operation.
The suspension system of an MRI magnet has to support the magnet mass, while providing adequate stiffness with minimal conduction heat leak. In addition to the mass and its dynamic load, an open architecture MRI suspension must support a large electromagnetic (EM) net force in each half of the magnet in the axial direction as well as a possible transverse EM force due to misalignment. The stiffness requirement in all directions is also more demanding in such an assembly in order to provide field stability under vibration.
The support problem has been overcome by a dual conical tortuous generally Z-shaped lengthened thermal path as discussed in more detail in the aformentioned related United States patent. However, such structures leave very little space between the warm suspension component and the cold thermal shield in which to position thermally insulation since the space varies from only .about.0.020 inches to .about.0.5". A preferred multi-layer insulation (hereinafter called "MLI") is thermal blankets which utilize alternating layers of a low conductivity spacer material between double sided aluminized mylar layers. However, in general, in the temperature range between 300K and 50K the performance of MLI blankets increases with the number of such layers. In fact, within that temperature range, it has been shown that even a few layers of MLI is better than using no MLI at all. However, performance of the MLI blanket is severely degraded due to increased heat transfer from direct conduction if the layers are compressed. This precludes installing blankets that have a natural or normal loft thickness greater than the space available. In addition, it is important to be able to properly position and retain in position such MLI blanket insulation.
Good heat interception in the suspension is essential to bring the 4 K heat load within the cooling capacity of the magnet, particularly if helium recondensing is provided since the mechanical cryocooler in a recondensing system is frequently at, or near, its cooling capacity.
All of the overlapping and conflicting requirements must be satisfied for a practical and satisfactory open architecture MRI superconducting magnet structure of the type described.